Known in the art is a spark ignition type internal combustion engine provided with a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve timing mechanism able to control a closing timing of a an intake valve, performing a supercharging action by a supercharger at the time of engine medium load operation and engine high load operation, and increasing the mechanical compression ratio and delaying the closing timing of the intake valve as the engine load becomes lower at the time of engine medium and high load operation in the state holding the actual compression ratio constant (for example, see Japanese Patent Publication (A) No. 2004-218522).
However, in a spark ignition type internal combustion engine, the state of the gas in the combustion chamber at the end of the compression stroke right before combustion, for example, the pressure in the combustion chamber or gas temperature, has a large effect on the combustion. That is, generally speaking, the higher the pressure in the combustion chamber or gas temperature at the end of the compression stroke, the easier the combustion, but the higher the pressure in the combustion chamber or gas temperature, the more knocking ends up occurring. Therefore, the pressure in the combustion chamber or gas temperature is preferably maintained at the optimal value, that is, the highest value as possible in the range where knocking does not occur.
On the other hand, if the actual compression ratio is held constant as in the above known internal combustion engine, the suction gas fed into the combustion chamber is compressed by a constant ratio at all times. However, in this case, the pressure in the combustion chamber or gas temperature at the end of the compression stroke changes depending on the pressure in the combustion chamber or gas temperature at the time of start of compression, that is, the pressure or temperature of the suction gas fed into the combustion chamber. If the pressure or temperature of the suction gas fed into the combustion chamber becomes higher, the pressure in the combustion chamber or temperature at the end of the compression stroke also becomes higher along with this. Therefore, there is the problem that even if maintaining the actual compression ratio constant as in the above known internal combustion engine, the pressure in the combustion chamber or gas temperature at the end of the compression stroke cannot be held at the optimum value.